1. Field of the Invention
The present invention relates to an impedance circuit having negative impedance.
2. Description of the Related Art
There have been demands for an impedance circuit having negative impedance. Particularly, a one-port circuit capable of varying resistance values from a negative resistance to a positive resistance. This one-port circuit cannot be a passive circuit constituted only by passive elements but has to be an active circuit including active elements such as transistors, FETs, and the like.
An example where negative impedance is required Will now be described.
In case of supplying electric power to a load from a direct current power supply, the potential difference between both ends of a load is requested to be maintained constant under a condition that the load current varies. If a direct current power supply having a remote sensing function is used in this case, the potential difference at the portion (sensing point) to which a sensing pair wire as a pair wire of a power supply for sensing is connected can be controlled accurately.
FIG. 8 is a circuit diagram showing an example of a power supply device having the remote sensing function. This direct current power supply device has a power supply section 101, a load 102, a resistor 103 which represents the resistance of a current path between a power supply and a sensing point, and a resistor 104 which represents another current path between the sensing point and the load. The resistance value of the resistor 103 is RL1. The resistance value of the resistor 104 is RL2. The current flowing through the resistor 104 is I load.
In this kind of power supply device, the output of the power supply sometimes causes an oscillation phenomenon when remote sensing is carried out. Furthermore, the sensing pair wire cannot always be connected near the load due to some reasons, such as mechanical interferences and physical limitations. In those cases, a voltage drop occurs due to the resistance RL2 which the current path between the sensing point and the load as shown in FIG. 8 has.
If the above-mentioned voltage drop caused between the sensing point and the load can be compensated, the voltage between the both ends of the load can be maintained constant. Accordingly, the problem as described above can be solved.
There also have been demands for a power supply which simulates a battery. For example, in a device which works with electric power supplied from a battery, test items concerning interactions between the device and the battery have been requested to be properly carried out before shipping.
Performance of a battery which works based on electrochemical effects deteriorates gradually as charging and discharging are repeated. Another factor causing deterioration in performance is repetition of a temperature cycle. Deterioration in performance based on aging of a battery built in a device appears as an increase in internal resistance of the battery. To become a power supply capable of simulating these characteristics of a battery, the power supply has to have an output resistance characteristic and a voltage characteristic substantially corresponding to the characteristic of a battery. If tests can be carried out using a power supply in place of a battery, data concerning a device to be tested can be accurately collected under critical operating conditions.
For example, in case of a battery of a cellular phone, a large current flows out of the battery when the cellular phone transmit. Consequently, the voltage of the battery lowers. At this time, if the voltage of the battery decreases lower than a lower limit of guaranteed operating conditions, the transmission is interrupted. Further, due to aging of the battery, the internal resistance of the battery increases and the voltage of the battery decreases at the time of outputting a large current. This increases the frequency at which transmission is interrupted.
Thus, the output resistance value of the power supply is requested to be varied smoothly, in order to simulate the internal resistance of a battery.
For example, the U.S. Pat. No. 6,459,247 has been known as a conventional technique relevant to the present invention. According to this patent document, a power supply having negative output resistance can be realized.
If a power supply having negative output resistance as disclosed in the above patent document is available, the voltage between two ends of a load can be maintained constant by compensating for a voltage drop caused between the sensing pair wire and the load as described above. This kind of power supply is expensive and has not yet been prevailing very much.